Pilot-assisted data transmission in a coherent optical-communication system

ABSTRACT

In one embodiment, an optical transmission system transmits data using a format according to which a data frame has two or more pilot-symbol blocks, each having a guard interval, and two or more payload-symbol blocks that are concatenated without a guard interval between them. The use of guard intervals in the pilot-symbol blocks helps the synchronization and channel-estimation procedures performed at a receiver of the optical transmission system to be robust in the presence of certain transmission impairments. The absence of guard intervals in the payload-symbol blocks helps to minimize the transmission overhead and thus achieve relatively high payload-data throughput. Pilot-symbol blocks have a structure that enables the receiver to determine channel-response characteristics for each data frame and then apply appropriate channel-response-compensation procedures to signals corresponding to the payload-symbol blocks of the frame to recover, with a relatively low bit-error rate, the data encoded in those signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject matter of this application is related to that of U.S. PatentApplication Publication No. 2012/0148264, which is incorporated hereinby reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to optical communication equipment and,more specifically but not exclusively, to signal processing in coherentoptical transport systems.

2. Description of the Related Art

This section introduces aspects that may help facilitate a betterunderstanding of the invention(s). Accordingly, the statements of thissection are to be read in this light and are not to be understood asadmissions about what is in the prior art or what is not in the priorart.

The next-generation of optical communication systems is being designedfor relatively high data-transmission rates, e.g., higher than about 100Gbit/s per channel. At these rates, the effects of chromatic dispersion(CD) and polarization-mode dispersion (PMD) significantly degrade thetransmission performance of optical fiber links. Since practicalimplementation of dispersion compensation in the optical domain isrelatively expensive, various digital-signal-processing (DSP)techniques, such as orthogonal frequency division multiplexing (OFDM),are used to mitigate the adverse effects of CD and PMD on signaltransmission. However, one problem with optical OFDM is that it requiresrelatively sophisticated digital signal processing not only at thereceiver, but also at the transmitter. In addition, optical OFDM has arelatively high peak-to-average power ratio (PAPR), which adverselyaffects the hardware cost by imposing rather stringent constraints onmodulation and power-amplifier nonlinearities.

SUMMARY

Disclosed herein are various embodiments of an optical transport systemthat transmits data using a format according to which a data frame hastwo or more pilot-symbol blocks, each having a cyclic prefix or suffix,and one or more payload-symbol blocks, each of which is concatenatedwith an adjacent block without a guard interval between them. The use ofcyclic prefixes/suffixes in the pilot-symbol blocks helps thesynchronization and channel-estimation procedures performed at areceiver of the optical transmission system to be robust in the presenceof transmission impairments, such as chromatic dispersion (CD) and/orpolarization-mode dispersion (PMD). The absence of guard intervals inthe payload-symbol blocks helps to minimize the transmission overheadand thus achieve relatively high payload-data throughput. Pilot-symbolblocks generated by the transmitter have a structure that enables theintended receiver to determine channel-response characteristics for eachdata frame so as to then apply appropriate channel-response-compensationprocedures to signals corresponding to the payload-symbol blocks of thedata frame to recover, with a relatively low bit-error rate, the dataencoded in those signals.

According to one embodiment, provided is an optical transmitter having adigital signal processor that converts a first data stream into a firstsequence of constellation symbols and generates a data frame comprisinga first plurality of pilot-symbol blocks and a first set of one or morepayload-symbol blocks. Each pilot-symbol block comprises a respectiveplurality of pilot symbols and a respective guard interval. Eachpayload-symbol block comprises a respective plurality of constellationsymbols from the first sequence. At least one payload-symbol block andanother block are concatenated without a guard interval between them,wherein the other block is either a pilot-symbol block of the same dataframe or of the next data frame or a payload-symbol block of the samedata frame. The optical transmitter further has an optical modulatorthat modulates an optical carrier to generate a modulated optical signalfor transmission of the data frame over an optical communication link.

According to another embodiment, provided is an optical transport systemhaving an optical transmitter and an optical receiver coupled to eachother via an optical communication link. In operation, the opticaltransmitter converts a first data stream into a first sequence ofconstellation symbols and generates a data frame that comprises a firstplurality of pilot-symbol blocks and a first set of one or morepayload-symbol blocks. Each pilot-symbol block comprises a respectiveplurality of pilot symbols and a respective guard interval. Eachpayload-symbol block comprises a respective plurality of constellationsymbols from the first sequence. At least one payload-symbol block andanother block are concatenated without a guard interval between them,wherein the other block is either a pilot-symbol block or apayload-symbol block. The optical transmitter modulates an opticalcarrier to generate a modulated optical signal having encoded thereonthe data frame and applies said modulated optical signal to the opticalcommunication link. The optical receiver receives the modulated opticalsignal from the optical communication link and converts it into a firstin-phase digital signal and a first quadrature-phase digital signal thathave a first set of signal samples corresponding to the data frame. Theoptical receiver then processes the first set of signal samples torecover the data encoded in the first sequence of constellation symbols.

According to yet another embodiment, provided is an opticalcommunication method having the steps of: converting a first data streaminto a first sequence of constellation symbols; and generating a dataframe that comprises a first plurality of pilot-symbol blocks and afirst set of one or more payload-symbol blocks. Each pilot-symbol blockcomprises a respective plurality of pilot symbols and a respective guardinterval. Each payload-symbol block comprises a respective plurality ofconstellation symbols from the first sequence. At least onepayload-symbol block and another block are concatenated without a guardinterval between them, wherein the other block is either a pilot-symbolblock or a payload-symbol block. The method further has the step ofmodulating an optical carrier to generate a modulated optical signal fortransmission of the data frame over an optical communication link.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and benefits of various embodiments of theinvention will become more fully apparent, by way of example, from thefollowing detailed description and the accompanying drawings, in which:

FIG. 1 shows a block diagram of an optical transmission system accordingto one embodiment of the invention;

FIGS. 2A-2C illustrate the operation of a digital signal processor thatcan be used in the transmitter of the optical transmission system shownin FIG. 1 according to one embodiment of the invention; and

FIGS. 3A-3C illustrate the operation of a digital signal processor thatcan be used in the receiver of the optical transmission system shown inFIG. 1 according to one embodiment of the invention.

DETAILED DESCRIPTION

OFDM transmission and single-carrier (SC) transmission withfrequency-domain equalization (FDE) have similardigital-signal-processing (DSP) complexities. The main DSP differencebetween these two signal transmission techniques is that, in OFDMtransmission, the inverse fast-Fourier-transform (IFFT) operation isperformed at the transmitter side while, in SC-FDE transmission, it isperformed at the receiver side. As a result, a transmitter in an SC-FDEtransmission system may use a digital-signal processor of lesserprocessing power than that of a digital-signal processor used in an OFDMtransmitter. In addition, an SC-FDE transmission system canadvantageously be configured to have a smaller peak-to-average powerratio (PAPR) value than a comparably performing OFDM system, therebyrelaxing the constraints on modulation and power-amplifiernonlinearities.

FIG. 1 shows a block diagram of a single-carrier optical transmissionsystem 100 according to one embodiment of the invention. System 100 hasan optical transmitter 110 and an optical receiver 190 connected via afiber link 150. In one embodiment, fiber link 150 is an amplified fiberlink having one or more optical amplifiers (not explicitly shown in FIG.1).

Transmitter 110 receives two independent data streams 102 and 104 fortransmission to receiver 190. A digital-signal processor 120 processesdata streams 102 and 104 as further described below in reference toFIGS. 2A-2C to generate digital signals 122 ₁-122 ₄. Digital signals 122₁-122 ₄ undergo a digital-to-analog conversion in digital-to-analogconverters (DACs) 124 ₁-124 ₄, respectively, to produce drive signals126 ₁-126 ₄. Drive signals 126 ₁ and 126 ₂ are in-phase (I) andquadrature-phase (Q) drive signals, respectively, corresponding to datastream 102. Drive signals 126 ₃ and 126 ₄ are similar in-phase andquadrature-phase drive signals corresponding to data stream 104.

An optical IQ modulator 140 _(X) uses drive signals 126 ₁ and 126 ₂ tomodulate an optical-carrier signal 132 _(X) generated by a laser source130 and to produce a modulated signal 142 _(X). An optical IQ modulator140 _(Y) similarly uses drive signals 126 ₃ and 126 ₄ to modulate anoptical-carrier signal 132 _(Y) generated by laser source 130 and toproduce a modulated signal 142 _(Y). A polarization beam combiner 146combines modulated signals 142 _(X) and 142 _(Y) to produce an opticalpolarization-division-multiplexed (PDM) signal 148. Note thatoptical-carrier signals 132 _(X) and 132 _(Y) have the same carrierfrequency. Each of drive signals 126 can be amplified by an RF amplifier(not explicitly shown) before being applied to drive the correspondingoptical IQ modulator 140.

Fiber link 150 receives signal 148 from beam combiner 146 fortransmission to receiver 190. While propagating through fiber link 150,signal 148 is subjected to various transmission impediments, such aschromatic dispersion (CD) and polarization mode dispersion (PMD), andemerges at the receiver end of the fiber link as an optical signal 152.

Receiver 190 has an optical-to-electrical (O/E) converter 160 having (i)two input ports labeled S and R and (ii) four output ports labeled 1through 4. Input port S receives optical signal 152. Input port Rreceives an optical reference signal 158 generated by an optical localoscillator (OLO) 156. Reference signal 158 has substantially the sameoptical-carrier frequency (wavelength) as signal 152. Reference signal158 can be generated, e.g., using a tunable laser controlled by awavelength-control loop (not explicitly shown in FIG. 1) that forces anoutput wavelength of the tunable laser to substantially track thecarrier wavelength of signal 152. In various embodiments, optical localoscillator 156 may comprise a combination of tunable and/or non-tunablelasers, optical frequency converters, optical modulators, and opticalfilters appropriately connected to one another to enable the generationof reference signal 158.

O/E converter 160 mixes input signal 152 and reference signal 158 togenerate eight mixed optical signals (not explicitly shown in FIG. 1).O/E converter 160 then converts the eight mixed optical signals intofour electrical signals 162 ₁-162 ₄ that are indicative of complexvalues corresponding to the two orthogonal-polarization components ofsignal 152. For example, electrical signals 162 ₁ and 162 ₂ may be ananalog in-phase signal and an analog quadrature-phase signal,respectively, corresponding to an x-polarization component of signal152. Electrical signals 162 ₃ and 162 ₄ may similarly be an analogin-phase signal and an analog quadrature-phase signal, respectively,corresponding to a y-polarization component of signal 152.

In one embodiment, O/E converter 160 is a polarization-diverse 90-degreeoptical hybrid (PDOH) with four balanced photo-detectors coupled to itseight output ports. Various suitable PDOHs are commercially available,e.g., from Optoplex Corporation of Fremont, Calif., and CeLight, Inc.,of Silver Spring, Md. Additional information on various O/E convertersthat can be used to implement O/E converter 160 in various embodimentsof system 100 are disclosed, e.g., in U.S. Patent ApplicationPublication No. 2010/0158521, U.S. patent application Ser. No.12/541,548 (filed on Aug. 14, 2009), and International PatentApplication No. PCT/US09/37746 (filed on Mar. 20, 2009), all of whichare incorporated herein by reference in their entirety.

Each of electrical signals 162 ₁-162 ₄ generated by O/E converter 160are converted into digital form in a corresponding one ofanalog-to-digital converters (ADCs) 166 ₁-166 ₄. Optionally, each ofelectrical signals 162 ₁-162 ₄ may be amplified in a correspondingamplifier (not explicitly shown) prior to the resulting signal beingconverted into digital form. Digital signals 168 ₁-168 ₄ produced byADCs 166 ₁-166 ₄ are processed by a digital signal processor 170, e.g.,as further described below in reference to FIGS. 3A-3C, to recover thedata applied by data streams 102 and 104 to transmitter 110. Therecovered data are outputted from receiver 190 via output signals 192and 194, respectively.

FIGS. 2A-2C illustrate the operation of a digital signal processor 200that can be used to implement digital signal processor 120 (FIG. 1)according to one embodiment of the invention. More specifically, FIG. 2Ashows a block diagram of processor 200. FIGS. 2B-2C show an exemplaryframe structure used by digital signal processor 200.

Processor 200 processes an input data stream 202 to generate digitaloutput signals 222 ₁ and 222 _(Q). In a representative embodiment,processor 120 (FIG. 1) is implemented using two processors 200configured to operate in parallel to one another. More specifically, forthe first of these two parallel processors 200, input data stream 202 isdata stream 102, and digital output signals 222 ₁ and 222 _(Q) aredigital signals 122 ₁ and 122 ₂, respectively. For the second of thesetwo parallel processors 200, input data stream 202 is data stream 104,and digital output signals 222 ₁ and 222 _(Q) are digital signals 122 ₃and 122 ₄, respectively.

Input data stream 202 is applied to a coding module 204, where it isoptionally interleaved and subjected to forward-error-correction (FEC)coding.

A coded bit stream 206 produced by coding module 204 is applied to aconstellation-mapping module 208, where it is converted into acorresponding sequence 210 of constellation symbols. The constellationused by constellation-mapping module 208 can be, for example, a QAM(Quadrature Amplitude Modulation) constellation or a QPSK (QuadraturePhase Shift Keying) constellation.

Symbol sequence 210 is applied to a framing module 212, where it isconverted into a corresponding sequence 214 of data frames. Whenprocessor 200 is used to implement DSP 120 (FIG. 1), sequence 214consists of two parallel subsequences, one corresponding to the Xpolarization and the other corresponding to the Y polarization. Framesequence 214 produced by framing module 212 is then applied to apulse-shaping module 218, where it is converted into output signals 222₁ and 222 _(Q).

FIG. 2B shows an exemplary structure of frame sequence 214 generated byframing module 212. A representative frame 230 of frame sequence 214comprises a plurality of pilot-symbol blocks (PSs) and a plurality ofpayload-symbol blocks (DSs). For example, frame 230 has (i) pilot-symbolblocks PS_(s), PS_(c1), and PS_(c2) and (ii) payload-symbol blocks DS₁,DS₂, . . . , DS_(n). One important difference between a pilot-symbolblock PS and a payload-symbol block DS is that the former has a cyclicprefix (CP, also sometimes referred to as a guard interval) while thelatter does not. The use of cyclic prefixes in pilot-symbol blocks PShelps the synchronization and channel-estimation procedures performed atthe receiver, e.g., receiver 190 (FIG. 1), to be robust in the presenceof transmission impairments, such as CD and/or PMD. The nonuse of cyclicprefixes in payload-symbol blocks DS helps to minimize the transmissionoverhead and thus achieve relatively high payload-data throughput.Channel-compensation procedures applied to the CP-free payload-symbolblocks (blocks DS in FIG. 2B) rely on the channel information obtainedfrom pilot-symbol blocks PS_(c1) and PS_(c2) and are described in moredetail below in reference to FIGS. 3A-3C.

Pilot-symbol block PS_(s) is designed to aid the receiver, e.g.,receiver 190 (FIG. 1), in frequency estimation and framesynchronization. An exemplary pilot-symbol block PS_(s) comprises (i)cyclic prefix CP_(s) and (ii) symbol sequence E(n) having 2N symbols(i.e., n=1, 2, . . . 2N), in which the string that has the first Nsymbols is identical to the string that has the last N symbols. Cyclicprefix CP_(s) has a copy of the last L symbols of symbol sequence E(n).In mathematical terms, pilot-symbol block PS_(s) is expressed by Eq.(1):PS_(S) =[E(n=(2N−L+1):2N),E(n=1:2N)]  (1)where the “:” symbol indicates a range for n starting from the value tothe left of the symbol and ending with the value to the right of thesymbol.

In one embodiment, symbol sequence E(n) is constructed as follows.First, a set of 2N orthogonal (in the OFDM sense) frequencies isselected and consecutively numbered starting from the lowest and endingwith the highest frequency. Second, each of the odd-numbered frequenciesis assigned the amplitude of zero, and each of the even-numberedfrequencies is assigned a symbol that is randomly selected from a QPSKconstellation. Recall that a QPSK constellation consists of fourconstellation points positioned, with uniform angular spacing, on acircle that is centered on the origin of a complex plane. The result ofthis assignment is a set having 2N frequency-domain symbols, half ofwhich are zeros. Finally, an inverse fast-Fourier-transform (IFFT)operation is applied to this set of 2N frequency-domain symbols toarrive at symbol sequence E(n).

In another embodiment, symbol sequence E(n) is expressed by Eq. (2):E(n)=exp(−jπ(n−1)² /N)  (2)where n=1, 2, . . . , 2N.

By having two identical halves, symbol sequence E(n) can readily be usedfor autocorrelation to find the starting point of the frame, and to findthe frequency offset between carrier signals 132 and reference signal158. Suitable methods that can be used to perform autocorrelation-basedsynchronization are described, e.g., in an article by T. M. Schmidl andD. C. Cox entitled “Robust Frequency and Timing Synchronization forOFDM,” published in IEEE Transactions on Communications, Vol. 45, No.12, December 1997, pp. 1613-1621, which article is incorporated hereinby reference in its entirety.

In one embodiment, pilot-symbol block PS_(s) corresponding to theX-polarization (e.g., represented by modulated signal 142 _(X), FIG. 1)is the same as pilot-symbol block PS_(s) corresponding to theY-polarization (e.g., represented by modulated signal 142 _(Y), FIG. 1).In an alternative embodiment, the X- and Y-polarizations may usedifferent respective pilot-symbol blocks PS_(s).

FIG. 2C shows an exemplary structure of pilot-symbol blocks PS_(c1) andPS_(c2). Note that the pilot-symbol blocks PS_(c1) and PS_(c2)corresponding to the X-polarization may differ from the pilot-symbolblocks PS_(c1) and PS_(c2) corresponding to the Y-polarization.Pilot-symbol blocks PS_(c1) and PS_(c2) are designed to aid thereceiver, e.g., receiver 190 (FIG. 1), in channel estimation and channelcompensation.

Similar to pilot-symbol block PS_(s), each pilot-symbol block PS_(c) hasa cyclic prefix (CP) prepended to a respective (known, predetermined)body sequence of symbols (KS). Cyclic prefix CP is constructed by takingseveral symbols from the end of body sequence KS, which makes eachpilot-data set PS_(c) a partially cyclic sequence. The length (L) ofcyclic prefix CP is selected to be longer than the expected duration ofthe impulse response of the channel, e.g., fiber link 150, FIG. 1. Thisproperty of pilot-symbol blocks in frame sequence 214 enables theintended receiver to process the pilot-symbol blocks in the receivedsignal (e.g., signal 152, FIG. 1) in a manner that mitigates inter-blockinterference imposed by the fiber link due to the effects of CD and PMD.

Body sequences KS corresponding to pilot-symbol blocks PS_(c1) andPS_(c2) have the same length, which is larger than the length of apayload-symbol block DS in one embodiment (also see the description ofFIG. 3C below). In a representative implementation, each body sequenceKS is a special polyphase sequence of length 2N, wherein the amplitudesof all non-zero symbols in the time domain have different phases but thesame amplitude, where N is a positive integer. One purpose of using thistype of a sequence is to enable the receiver to accurately and uniformlyprobe the channel-response function, H, over the entire frequency rangeof interest. Channel-response function H is a frequency-dependent 2×2matrix whose elements are complex functions of frequency that describethe combined signal-transfer characteristics of the front end of thetransmitter (e.g., transmitter 110, FIG. 1), the fiber link (e.g., fiberlink 150, FIG. 1), and the front end of the receiver (e.g., receiver190, FIG. 1).

In one embodiment, each body sequence KS has 2N symbols, and theindividual body sequences KS₁-KS₄ shown in FIG. 2C are expressed by Eqs.(3)-(5):KS ₁(n)=exp(−jπ(n−1)² /N)  (3)KS ₂(n)=exp(−jπn(n−1)/N)  (4)KS ₃ =KS ₂ , KS ₄ =KS ₁  (5)where n=1, 2, . . . , 2N. The relationship between different bodysequences KS expressed by Eq. (5) may be advantageous in that it enablesthe receiver to readily calculate, in the frequency domain, all fourelements of the 2×2 channel-response matrix as a function of frequency.A suitable matrix-calculation method that can be used for this purposeis described, e.g., in an article by C. J. Youn, “An Efficient andFrequency-Offset-Tolerant Channel Estimation and Synchronization Methodfor PDM CO-OFDM Transmission,” published in the 2010 European Conferenceon Optical Communications (ECOC '10) as paper P4.06.

In alternative embodiments, additional suitable polyphase sequences canbe constructed from the polyphase sequences defined by Eqs. (3)-(4),e.g., by one or more of the following: (i) cyclically shifting theentire sequence; (ii) phase shifting each symbol of the sequence by aconstant phase; (iii) taking the m-th power of each symbol of thesequence, where m is an integer greater than one; and (iv)phase-conjugating the entire sequence. Additional suitable polyphasesequences (or codes) with uniform amplitudes in both the time andfrequency domains can be constructed based on the general descriptionprovided in the article by David C. Chu, “Polyphase Codes with GoodPeriodic Correlation Properties,” published in IEEE Transactions onInformation Theory, July 1972, pp. 531-532, which is incorporated hereinby reference in its entirety.

Note that pilot-symbol blocks PS_(c1) and PS_(c2) corresponding to the Xpolarization are synchronous with pilot-symbol blocks PS_(c1) andPS_(c2), respectively, corresponding to the Y polarization. Thischaracteristic of pilot-symbol blocks PS_(c1) and PS_(c2) is amanifestation of a more-general characteristic of data frame 230,according to which each symbol block corresponding to the X polarizationis synchronous with a counterpart symbol block corresponding to the Ypolarization. As such, pilot-symbol blocks PS_(s) corresponding to the Xpolarization is synchronous with pilot-symbol block PS_(s) correspondingto the Y polarization. Similarly, each payload-symbol block DS_(i)corresponding to the X polarization is synchronous with pilot-symbolblock DS_(i) corresponding to the Y polarization (see FIG. 2B).

Referring back to FIG. 2A, frame sequence 214 produced by framing module212 is applied to a pulse-shaping module 218, where it is converted intooutput signals 222 _(I) and 222 _(Q). Pulse shaping implemented inpulse-shaping module 218 is a process of generating a digital waveformthat, after being converted into a corresponding analog signal, can beapplied to an optical modulator to modulate an optical-carrier signal,such as optical-carrier signal 132 _(X) or 132 _(Y) (FIG. 1), so thatthe resulting modulated optical signal is modulated with symbolscorresponding to the various blocks of frame sequence 214. In oneconfiguration, oversampling may be applied, e.g., by duplicating eachsignal sample one or more times for output signals 222 _(I) and 222_(Q).

FIGS. 3A-3C illustrate the operation of a digital signal processor 300that can be used to implement digital signal processor 170 (FIG. 1)according to one embodiment of the invention. More specifically, FIGS.3A-3B show block diagrams of processor 300. FIG. 3C shows the processingimplemented in an FDCCE(frequency-domain-channel-compensation/equalization) sub-module 352 ofprocessor 300. When processor 300 is used as processor 170, inputsignals 302 ₁-302 ₄ correspond to signals 168 ₁-168 ₄ (FIG. 1),respectively, and output signals 332 _(x) and 332 _(y) correspond tooutput signals 192 and 194 (FIG. 1), respectively.

EDC (electronic dispersion compensation) modules 310 perform digitalsignal processing that mitigates the detrimental effects of chromaticdispersion imposed on input signal 152 by fiber link 150. In particular,EDC module 310 _(x) processes input signals 302 ₁-302 ₂ corresponding tothe first principal polarization axis (e.g., the x axis) of PDOH 160(FIG. 1). Similarly, EDC module 310 _(y) processes input signals 302₃-302 ₄ corresponding to the second principal polarization axis (e.g.,the y axis) of PDOH 160. Note that the X and Y polarizations used at thetransmitter may or may not be aligned with the principal polarizationaxes (i.e., the x and y axes) of PDOH 160. Various EDC modules that canbe used to implement EDC module 310 are disclosed, e.g., in U.S. Pat.Nos. 7,570,889, 7,532,820, and 7,382,984, all of which are incorporatedherein by reference in their entirety.

Dispersion-compensated signals 312 ₁-312 ₄ produced by EDC modules 310_(x) and 310 _(y) are applied to a pilot-assistedfrequency-division-equalization and decoding (PA-FDED) module 320 thatprocesses these signals, e.g., as further described below in referenceto FIGS. 3B-3C, to recover the original data streams applied to thetransmitter for transmission, e.g., data streams 102 and 104 of FIG. 1.More specifically, PA-FDED module 320 generates data stream 332 _(x)that carries the data corresponding to the first independently modulatedcomponent of signal 152 (e.g., component 142 _(X), FIG. 1), andsimilarly generates data stream 332 _(y) that carries the datacorresponding to the second independently modulated component of signal152 (e.g., component 142 _(Y), FIG. 1).

FIG. 3B shows a block diagram of PA-FDED module 320 according to oneembodiment of the invention. PA-FDED module 320 has a synchronizationsub-module 340 that receives, as its input, dispersion-compensatedsignals 312 ₁-312 ₄. Synchronization sub-module 340 relies on theabove-described properties of pilot-symbol blocks PS_(s) to determinethe start of each frame 230 (also see FIG. 2B). In one configuration,synchronization sub-module 340 calculates an autocorrelation functiondefined by Eq. (6):

$\begin{matrix}{{P(n)} = {\sum\limits_{k = 1}^{N}\lbrack {{{r_{x}( {n + k - 1} )}*{r_{x}( {n + k + N - 1} )}} + {{r_{y}( {n + k - 1} )}*{r_{y}( {n + k + N - 1} )}}} \rbrack}} & (6)\end{matrix}$where the “*” symbol denotes a complex conjugate; r_(x)(n) is a complexvalue whose real part is a signal sample provided by signal 312 ₁ andwhose imaginary part is a corresponding signal sample provided by signal312 ₂; and r_(y)(n) is a complex value whose real part is a signalsample provided by signal 312 ₃ and whose imaginary part is acorresponding signal sample provided by signal 312 ₄. Since symbolsequence E(n) of pilot-symbol block PS_(s) has two identical portions oflength N, the absolute value of function P(n) has a pronounced maximumthat is temporally aligned with the first symbol of symbol sequence E(n)and, as such, can be used to determine the temporal position of thecorresponding frame 230.

An FE (frequency-estimation/compensation) sub-module 344 that is locateddownstream from synchronization sub-module 340 performs electronicestimation and compensation of a mismatch between the carrier-frequencyof input signal 152 and the frequency of reference signal 158 (see FIG.1). In one embodiment, FE sub-module 344 determines the phase ofP(n_(max)), where n_(max) is the time slot corresponding to the maximumof the absolute value of function P(n) (see Eq. (6)) determined bysynchronization sub-module 340. FE sub-module 344 then uses the phase ofP(n_(max)) to calculate the frequency offset between signals 152 and158. If the frequency offset is zero, then P(n_(max)) is real and itsphase is zero. If the frequency offset is not zero, then P(n_(max)) iscomplex and its phase is directly related to the frequency offset andthe duration of pilot-symbol block PS_(s). Because the frequency offsetmay change over time, FE sub-module 344 performs the frequency-offsetcalculation for each received frame 230.

After the frequency offset is determined, FE sub-module 344 performsfrequency-mismatch compensation by applying to each signal sample aphase shift equal to the frequency offset multiplied by 2π and the timeelapsed between the start of the frame determined by synchronizationsub-module 340 and the signal sample. Various FE modules that can beadapted to function as FE sub-module 344 are disclosed, e.g., in U.S.Pat. No. 7,747,177 and U.S. Patent Application Publication No.2008/0152361, both of which are incorporated herein by reference intheir entirety.

A CE (channel-estimation) sub-module 348 uses signal samplescorresponding to pilot-symbol blocks PS_(c1) and PS_(c2) to determinethe channel-response function, H, which can be expressed in thefrequency domain as a 2×2 Jones matrix given by Eq. (7):

$\begin{matrix}{{H(f)} = \begin{pmatrix}{a(f)} & {b(f)} \\{c(f)} & {d(f)}\end{pmatrix}} & (7)\end{matrix}$More specifically, using the a priori knowledge of body sequences KS₁,KS₂, KS₃, and KS₄, CE sub-module 348 constructs channel-responsefunction H so that the application of that channel-response function tothese body sequences transforms them into the received signal samplescorresponding to these known body sequences. Note that an individualchannel-response function H can be calculated by CE sub-module 348 foreach frame 230, which enables the receiver to appropriately tracktime-dependent channels.

FIG. 3C also shows exemplary processing implemented in FDCCE(frequency-domain channel-compensation/equalization) sub-module 352.More specifically, the shown processing handles signal samplescorresponding to the payload of a single transmitted frame (e.g.,payload-symbol blocks DS in frame 230 having data corresponding to boththe X and Y polarizations, see FIG. 2B) and, as such, uses thechannel-response function H determined by channel-estimation sub-module348. For processing each new frame, FDCCE sub-module 352 uses acorresponding new channel-response function H determined by and receivedfrom CE sub-module 348.

As already indicated above, the symbols of payload-symbol blocks DS inframe 230 do not have cyclic prefixes. As a result, inter-blockinterference (IBI) occurs at the receiver. To mitigate the adverseeffects of IBI, FDCCE sub-module 352 performs, as further describedbelow and with respect to FIG. 3C, overlap FDCCE processing with asliding window 368 that has 2N consecutive signal samples from asequence 366 of frequency-corrected signal samples corresponding to thepayload of a received frame. Sequence 366 consists of two parallelsub-sequences, i.e., a sequence 366 _(x) that corresponds to signals 312₁-312 ₂ and a sequence 366 _(y) that corresponds to signals 312 ₃-312 ₄(also see FIGS. 3A-3B). Note that signal samples in sequence 366 _(x)have contributions both from the symbols carried by signal 142 _(x) andfrom the symbols carried by signal 142 _(Y) (FIG. 1). Signal samples insequence 366 _(y) similarly have contributions both from the symbolscarried by signal 142 _(X) and from the symbols carried by signal 142_(Y) (FIG. 1).

For an i-th position of sliding window 368, FDCCE sub-module 352applies: (i) a fast Fourier transform (FFT) 372 _(ix) to a block 370_(ix) of 2N signal samples located inside the window and belonging tosequence 366 _(x) and (ii) an FFT 372 _(iy) to a block 370 _(iy) of 2Nsignal samples located inside the window and belonging to sequence 366_(y). FFT operation 372 _(ix) converts block 370 _(ix) into acorresponding block 374 _(ix) of 2N frequency components. FFT operation374 similarly converts block 370 _(iy) into a corresponding block 374_(iy) of 2N frequency components.

Blocks 374 _(ix) and 374 _(iy) are subjected to afrequency-domain-equalization (FDE) procedure 376 _(i), which convertsthese blocks into blocks 378 _(ix) and 378 _(iy). Similar to blocks 374_(i), each of blocks 378 _(ix) and 378 _(iy) has 2N frequencycomponents. FDE procedure 376 _(i) includes the application of inversechannel-response function H⁻¹, which is derived by FDCCE sub-module 352from channel-response function H (see Eq. (7)) determined bychannel-estimation sub-module 348. Since channel-response function Hgenerally has a non-diagonal form (i.e., b(ƒ)≠0 and c(ƒ)≠0), FDEprocedure 376; mixes frequency components from blocks 374 _(ix) and 374_(iy) to produce the corresponding frequency components for blocks 378_(ix) and 378 _(iy). Note that FFT operations 372 _(i) do not have thisfeature because (i) block 374 _(ix) is produced by operating solely onblock 370 _(ix) and without using any signal samples from block 370_(iy) and (ii) block 374 _(iy) is produced by operating solely on block370 _(iy) and without using any signal samples from block 370 _(ix).

Blocks 378 _(ix) and 378 _(iy) are subjected to inverse FFT (IFFT)operations 380 _(ix) and 380 _(iy), respectively. IFFT operation 380_(ix) converts block 378 _(ix) into 2N time-domain signal samples, whichform block 382 _(ix). IFFT operation 380 _(iy) similarly converts block378 _(iy) into 2N time-domain signal samples, which form block 382_(iy). Similar to FFT operations 372 _(i), IFFT operations 380 _(i) donot mix the x and y blocks.

Block 382 _(ix) is truncated to remove N_(e) signal samples from thebeginning of the block and N_(e) signal samples from the end of theblock, where N_(e) is a predetermined number chosen so that the signalsamples affected by IBI are being truncated out. The remaining portionof block 382 _(ix) having 2N-2N_(e) signal samples is used to formsequence 384 _(x) of equalized signal samples that is output from FDCCEsub-module 352 to the downstream sub-modules of PA-FDED module 320.Block 384 is similarly truncated to remove N_(e) signal samples from thebeginning of the block and N_(e) signal samples from the end of theblock. The remaining portion of block 384 having 2N-2N_(e) signalsamples is used to form sequence 384 _(y) of equalized signal samplesthat is also output from FDCCE sub-module 352 to the downstreamsub-modules of PA-FDED module 320.

Sliding window 368 is shifted down sequences 366 _(x) and 366 _(y) by2N-2N_(e) signal samples to the (i+1)-th position, and the processingthat is described above for the i-th is repeated for the (i+1)-thposition as indicated in FIG. 3C. The shifts of sliding window 368 by2N-2N_(e) signal samples and the corresponding processing are repeateduntil all of the payload symbols of the corresponding frame 230 for bothX- and Y-polarizations have been recovered.

In one configuration, N_(e)=L/2 and 2N−2N_(e)=N_(DS), where L is thelength of cyclic prefix CP in pilot-symbol blocks PS_(c1) and PS_(c2)(FIG. 2C), and N_(DS) is the length of a payload-symbol block (DS) inframe 230 (FIG. 2B). In this configuration, the processing performed atthe i-th position of sliding window 368 recovers all symbols ofpayload-symbol block DS_(i) in frame 230 (see FIG. 2B). To recover thesymbols of the first payload-symbol block in the frame (DS₁), slidingwindow 368 is positioned so that the first N_(e) signal samples insidethe window belong to pilot-symbol block PS_(c2) of the same frame. Torecover the symbols of the last payload-symbol block in the frame(DS_(n)), sliding window 368 is positioned so that the last N_(e) signalsamples inside the window belong to pilot-symbol block PS_(s) of thenext frame.

Referring back to FIG. 3B, equalized sequences 384 _(x) and 384 _(y)produced by FDCCE sub-module 352 are applied to aphase-estimation/phase-correction (PE/PC) sub-module 356. PE/PCsub-module 356 performs digital processing that corrects or compensatesfor slowly changing phase shifts between input signal 152 and referencesignal 158 (FIG. 1), and then estimates the phase of the signal samplesin equalized sequences 384 _(x) and 384 _(y) for constellation demappingand decoding. Various processing modules that can be used to implementPE/PC sub-module 356 are disclosed, e.g., in above-cited U.S. PatentApplication Publication No. 2008/0152361 and also in U.S. Pat. No.7,688,918 and U.S. Patent Application Publication No. 2008/0075472, bothof which are incorporated herein by reference in their entirety.

A demapping sub-module 360 uses the phase estimates obtained by PE/PCsub-module 356 and the constellation map to convert equalized sequences384 _(x) and 384 _(y) into the corresponding sequences of constellationsymbols. Demapping sub-module 360 then decodes each constellation symbolto convert it into the corresponding set of bits, thereby generating bitstreams 362 _(x) and 362 _(y), corresponding to equalized sequences 384_(x) and 384 _(y), respectively. In the absence of errors, bit stream362 _(x) is a copy of bit stream 206 _(x), and bit stream 362 _(y) is acopy of bit stream 206 _(Y) (also see FIG. 2A). When decoding errors arepresent, a bit stream 362 may differ somewhat from the corresponding bitstream 206.

An FEC (forward error correction) sub-module 364 performs errorcorrection in bit streams 362 _(x) and 362 _(y) using data redundanciesthat were introduced into the corresponding bit streams 206 by codingmodules 204 (FIG. 2A). The resulting error-corrected bit streams areoutput via signals 332 _(x) and 332 _(y). Many FEC methods suitable foruse in FEC sub-module 364 are known in the art. Both hard-decision andsoft-decision decoding may be implemented in various embodiments of FECsub-module 364. Several representative examples of such methods aredisclosed, e.g., in U.S. Pat. Nos. 7,734,191, 7,574,146, 7,424,651,7,212,741, and 6,683,855, all of which are incorporated herein byreference in their entirety.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Although various embodiments of the invention have beendescribed in reference to polarization-division-multiplexed (PDM)signals, the invention is not so limited and may be similarly applied toprocessing non-PDM signals. Various modifications of the describedembodiments, as well as other embodiments of the invention, which areapparent to persons skilled in the art to which the invention pertainsare deemed to lie within the principle and scope of the invention asexpressed in the following claims.

The term “single-carrier” is a term of art that is used in thisspecification to contrast embodiments of the optical transmission systemdisclosed herein with an optical OFDM transmission system. This termshould not be interpreted to imply that embodiments of the invention arenot compatible with the WDM technology. One of ordinary skill in the artwill appreciate that certain signal processing techniques disclosedherein may be applied to each of different carrier frequencies(wavelengths) of a WDM multiplex.

As used herein, the term “synchronous” refers to temporal alignment oftwo data or symbol blocks, two symbols, and/or the time slotscorresponding to them. For example, two symbol blocks are considered tobe synchronous if their leading edges arrive at a specified location(e.g., an input port or an output port) substantially simultaneously,i.e., the difference between the times of arrival is smaller than adesignated relatively small tolerance. Similarly, two symbols areconsidered to be synchronous if their leading edges arrive at aspecified location substantially simultaneously, i.e., the differencebetween the times of arrival is smaller than a designated relativelysmall tolerance.

Although various embodiments of the invention have been described inreference to cyclic prefixes, the invention is not so limited. Based onthe provided description, one skilled in the art will be able tosimilarly practice the invention with cyclic suffixes instead of or inaddition to cyclic prefixes. As used herein, the term “guard interval”should be interpreted as a general term that covers both cyclic prefixesand cyclic suffixes.

In various embodiments, each data frame has at least one payload-symbolblock (DS_(i), FIG. 2B) that is concatenated with another block withouta guard interval between them. For example, if data frames use cyclicprefixes as guard intervals, then one of such concatenation points islocated at the boundary between the last pilot-symbol block of a dataframe and the first payload-symbol block of the same data frame, e.g.,between pilot-symbol block PS_(c2) and payload-symbol block DS₁ in FIG.2B. Alternatively, if data frames use cyclic suffixes as guardintervals, then one of such concatenation points is located at theboundary between the last payload-symbol block of a data frame and thefirst pilot-symbol block of the next data frame, e.g., betweenpayload-symbol block DS_(n) and the adjacent pilot-symbol block PS_(s)in FIG. 2B.

The present invention may be implemented as circuit-based processes,including possible implementation on a single integrated circuit.

Unless explicitly stated otherwise, each numerical value and rangeshould be interpreted as being approximate as if the word “about” or“approximately” preceded the value of the value or range.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the scope of theinvention as expressed in the following claims.

The use of figure numbers and/or figure reference labels in the claimsis intended to identify one or more possible embodiments of the claimedsubject matter in order to facilitate the interpretation of the claims.Such use is not to be construed as necessarily limiting the scope ofthose claims to the embodiments shown in the corresponding figures.

Although the elements in the following method claims, if any, arerecited in a particular sequence with corresponding labeling, unless theclaim recitations otherwise imply a particular sequence for implementingsome or all of those elements, those elements are not necessarilyintended to be limited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

Also for purposes of this description, the terms “couple,” “coupling,”“coupled,” “connect,” “connecting,” or “connected” refer to any mannerknown in the art or later developed in which energy is allowed to betransferred between two or more elements, and the interposition of oneor more additional elements is contemplated, although not required.Conversely, the terms “directly coupled,” “directly connected,” etc.,imply the absence of such additional elements.

The embodiments covered by the claims in this application are limited toembodiments that (1) are enabled by this specification and (2)correspond to statutory subject matter. Non-enabled embodiments andembodiments that correspond to non-statutory subject matter areexplicitly disclaimed even if they formally fall within the scope of theclaims.

The description and drawings merely illustrate the principles of theinvention. It will thus be appreciated that those of ordinary skill inthe art will be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples recited herein are principally intended expressly to be onlyfor pedagogical purposes to aid the reader in understanding theprinciples of the invention and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass equivalents thereof.

The functions of the various elements shown in the figures, includingany functional blocks labeled as “processors,” may be provided throughthe use of dedicated hardware as well as hardware capable of executingsoftware in association with appropriate software. When provided by aprocessor, the functions may be provided by a single dedicatedprocessor, by a single shared processor, or by a plurality of individualprocessors, some of which may be shared. Moreover, explicit use of theterm “processor” or “controller” should not be construed to referexclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (DSP)hardware, application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA), read only memory (ROM) for storingsoftware, random access memory (RAM), and non volatile storage. Otherhardware, conventional and/or custom, may also be included. Similarly,any switches shown in the figures are conceptual only. Their functionmay be carried out through the operation of program logic, throughdedicated logic, through the interaction of program control anddedicated logic, or even manually, the particular technique beingselectable by the implementer as more specifically understood from thecontext.

It should be appreciated by those of ordinary skill in the art that anyblock diagrams herein represent conceptual views of illustrativecircuitry embodying the principles of the invention. Similarly, it willbe appreciated that any flow charts, flow diagrams, state transitiondiagrams, pseudo code, and the like represent various processes whichmay be substantially represented in computer readable medium and soexecuted by a computer or processor, whether or not such computer orprocessor is explicitly shown.

What is claimed is:
 1. An optical transmitter, comprising: a digitalsignal processor configured to convert a first data stream into a firstsequence of constellation symbols and to generate a data frame thatcomprises: a first plurality of pilot-symbol blocks, wherein eachpilot-symbol block comprises a respective plurality of pilot symbols anda respective guard interval; and a first set of one or morepayload-symbol blocks, wherein each payload-symbol block comprises arespective plurality of constellation symbols from the first sequence;and an optical modulator configured to modulate an optical carrier togenerate a modulated optical signal for transmission of the data frameover an optical communication link, wherein: at least one payload-symbolblock and another block are concatenated without a guard intervalbetween them; the other block is either a pilot-symbol block or apayload-symbol block; and the first plurality of pilot-symbol blockscomprises: a first channel-estimation block having a first polyphasesequence of symbols; and a second channel-estimation block having asecond polyphase sequence of symbols different from the first polyphasesequence of symbols.
 2. The optical transmitter of claim 1, wherein thefirst set of one or more payload-symbol blocks comprises a plurality ofpayload-symbol blocks, wherein each pair of consecutive payload-symbolblocks in the first set is concatenated without a guard interval betweenthem.
 3. The optical transmitter of claim 1, wherein the first pluralityof pilot-symbol blocks comprises a frame-synchronization block having afirst string of N pilot symbols concatenated with a second string of Npilot symbols, said second string being a copy of the first string,where N is a positive integer.
 4. The optical transmitter of claim 1,wherein the symbols of the first polyphase sequence and the symbols ofthe second polyphase sequence have equal amplitudes.
 5. The opticaltransmitter of claim 1, wherein: the first polyphase sequence has aconstant amplitude and a phase pattern defined by phase sequence φ₁(n),which is expressed as:φ₁(n)=π(n−1)² /N; and the second polyphase sequence has a constantamplitude and a phase pattern defined by phase sequence φ₂(n), which isexpressed as:φ₂(n)=πn(n−1)/N, where n=1, 2, . . . , 2N and N is a positive integer.6. The optical transmitter of claim 1, wherein: the digital processor isfurther configured to convert a second data stream into a secondsequence of constellation symbols, wherein the data frame furthercomprises: a second plurality of pilot-symbol blocks, wherein eachpilot-symbol block comprises a respective plurality of pilot symbols anda respective guard interval; and a second set of one or morepayload-symbol blocks, wherein each payload-symbol block of the secondset comprises a respective plurality of constellation symbols from thesecond sequence; the optical modulator is configured to modulate a firstpolarization of the modulated optical signal with the symbols of thefirst plurality of pilot-symbol blocks and the first set of one or morepayload-symbol blocks; and the optical modulator is further configuredto modulate a second polarization of the modulated optical signal withthe symbols of the second plurality of pilot-symbol blocks and thesecond set of one or more payload-symbol blocks.
 7. The opticaltransmitter of claim 6, wherein: the first plurality of pilot-symbolblocks is synchronous with the second plurality of pilot-symbol blocks;and the first set of one or more payload-symbol blocks is synchronouswith the second set of one or more payload-symbol blocks.
 8. The opticaltransmitter of claim 6, wherein: the first plurality of pilot-symbolblocks comprises a first frame-synchronization block having a firststring of N pilot symbols concatenated with a second string of N pilotsymbols, said second string being a copy of the first string, where N isa positive integer; the second plurality of pilot-symbol blockscomprises a second frame-synchronization block having a third string ofN pilot symbols concatenated with a fourth string of N pilot symbols,said fourth string being a copy of the third string; and the thirdstring is different from the first string.
 9. The optical transmitter ofclaim 6, wherein: the first plurality of pilot-symbol blocks comprises:a first channel-estimation block having a first polyphase sequence ofsymbols; and a second channel-estimation block having a second polyphasesequence of symbols different from the first polyphase sequence ofsymbols; and the second plurality of pilot-symbol blocks comprises: athird channel-estimation block having the second polyphase sequence; anda fourth channel-estimation block having the first polyphase sequence,wherein: the first channel-estimation block is synchronous with thethird channel-estimation block; and the second channel-estimation blockis synchronous with the fourth channel-estimation block.
 10. An opticaltransport system, comprising: an optical transmitter; and an opticalreceiver coupled to the optical transmitter via an optical communicationlink, wherein: the optical transmitter is configured to convert a firstdata stream into a first sequence of constellation symbols and togenerate a data frame that comprises: a first plurality of pilot-symbolblocks, wherein each pilot-symbol block comprises a respective pluralityof pilot symbols and a respective guard interval; and a first set of oneor more payload-symbol blocks, wherein each payload-symbol blockcomprises a respective plurality of constellation symbols from the firstsequence; at least one payload-symbol block and another block areconcatenated without a guard interval between them, wherein the otherblock is either a pilot-symbol block or a payload-symbol block; theoptical transmitter is further configured to modulate an optical carrierto generate a modulated optical signal having encoded thereon the dataframe and to apply said modulated optical signal to the opticalcommunication link; the optical receiver is configured to receive themodulated optical signal from the optical communication link and toconvert the received modulated optical signal into a first in-phasedigital signal and a first quadrature-phase digital signal that have afirst set of signal samples corresponding to the data frame; the opticalreceiver is further configured to process the first set of signalsamples to recover the data encoded in the first sequence ofconstellation symbols; the optical transmitter is further configured toconvert a second data stream into a second sequence of constellationsymbols, wherein the data frame further comprises: a second plurality ofpilot-symbol blocks, wherein each pilot-symbol block comprises arespective plurality of pilot symbols and a respective guard interval;and a second set of one or more payload-symbol blocks, wherein eachpayload-symbol block of the second set comprises a respective pluralityof constellation symbols from the second sequence; the opticaltransmitter is further configured to modulate a first polarization ofthe modulated optical signal with the symbols of the first plurality ofpilot-symbol blocks and the first set of one or more payload-symbolblocks; the optical transmitter is further configured to modulate asecond polarization of the modulated optical signal with the symbols ofthe second plurality of pilot-symbol blocks and the second set of one ormore payload-symbol blocks; the first plurality of pilot-symbol blockscomprises a first frame-synchronization block having a first string of Npilot symbols concatenated with a second string of N pilot symbols, saidsecond string being a copy of the first string, where N is a positiveinteger; the second plurality of pilot-symbol blocks comprises a secondframe-synchronization block having a third string of N pilot symbolsconcatenated with a fourth string of N pilot symbols, said fourth stringbeing a copy of the third string; and the third string is different fromthe first string.
 11. The optical transport system of claim 10, wherein:the optical receiver is further configured to convert the receivedmodulated optical signal into a second in-phase digital signal and asecond quadrature-phase digital signal that have a second set of signalsamples corresponding to a data frame; and the optical receiver isfurther configured to process the first set of signal samples togetherwith the second set of signal samples to recover the data encoded bothin the first sequence of constellation symbols and in the secondsequence of constellation symbols.
 12. The optical transport system ofclaim 11, wherein: the first plurality of pilot-symbol blocks comprises:a first channel-estimation block having a first polyphase sequence ofsymbols; and a second channel-estimation block having a second polyphasesequence of symbols different from the first polyphase sequence ofsymbols; and the second plurality of pilot-symbol blocks comprises: athird channel-estimation block having the second polyphase sequence; anda fourth channel-estimation block having the first polyphase sequence,wherein: the first channel-estimation block is synchronous with thethird channel-estimation block; and the second channel-estimation blockis synchronous with the fourth channel-estimation block.
 13. An opticalcommunication method, comprising: converting a first data stream into afirst sequence of constellation symbols; generating a data frame thatcomprises: a first plurality of pilot-symbol blocks, wherein eachpilot-symbol block comprises a respective plurality of pilot symbols anda respective guard interval; and a first set of one or morepayload-symbol blocks, wherein each payload-symbol block comprises arespective plurality of constellation symbols from the first sequence;modulating an optical carrier to generate a modulated optical signal fortransmission of the data frame over an optical communication link,wherein: at least one payload-symbol block and another block areconcatenated without a guard interval between them; and the other blockis either a pilot-symbol block or a payload-symbol block; and generatinga frame-synchronization block for the first plurality of pilot-symbolblocks by: selecting 2N orthogonal frequencies, where N is a positiveinteger; consecutively numbering said frequencies starting from a lowestfrequency and ending with a highest frequency; assigning azero-amplitude symbol to each of the odd-numbered frequencies; assigninga symbol that is randomly selected from a QPSK constellation to each ofthe even-numbered frequencies; and applying an inversefast-Fourier-transform (IFFT) operation to a resulting set of 2Nfrequency-domain symbols to generate a string of time-domain symbols forthe frame-synchronization block.
 14. The method of claim 13, wherein thefirst plurality of pilot-symbol blocks comprises a channel-estimationblock having a first polyphase sequence of symbols, which has beengenerated from a second polyphase sequence of symbols using one or moreof the following: cyclically shifting the second polyphase sequence;phase shifting each symbol of the second polyphase sequence by aconstant phase; taking an m-th power of each symbol of the secondpolyphase sequence, where m is an integer greater than one; andphase-conjugating the second polyphase sequence.
 15. The method of claim14, wherein the second polyphase sequence has a constant amplitude and aphase pattern defined by phase sequence φ₁(n) or phase sequence φ₂(n),where φ₁(n)=−π(n−1)²/N; φ₂(n)=−πn(n−1)/N; n=1, 2, . . . , 2N; and N is apositive integer.
 16. The method of claim 13, further comprisingconverting a second data stream into a second sequence of constellationsymbols, wherein the data frame further comprises: a second plurality ofpilot-symbol blocks, wherein each pilot-symbol block comprises arespective plurality of pilot symbols and a respective guard interval;and a second set of one or more payload-symbol blocks, wherein: eachpayload-symbol block of the second set comprises a respective pluralityof constellation symbols from the second sequence; a first polarizationof the modulated optical is modulated with the symbols of the firstplurality of pilot-symbol blocks and the first set of one or morepayload-symbol blocks; and a second polarization of the modulatedoptical is modulated with the symbols of the second plurality ofpilot-symbol blocks and the second set of one or more payload-symbolblocks.
 17. The method of claim 16, wherein: the first plurality ofpilot-symbol blocks comprises: a first channel-estimation block having afirst polyphase sequence of symbols; and a second channel-estimationblock having a second polyphase sequence of symbols different from thefirst polyphase sequence of symbols; and the second plurality ofpilot-symbol blocks comprises: a third channel-estimation block havingthe second polyphase sequence; and a fourth channel-estimation blockhaving the first polyphase sequence, wherein: the firstchannel-estimation block is synchronous with the thirdchannel-estimation block; and the second channel-estimation block issynchronous with the fourth channel-estimation block.
 18. An opticaltransmitter, comprising: a digital signal processor configured toconvert a first data stream into a first sequence of constellationsymbols and to generate a data frame that comprises: a first pluralityof pilot-symbol blocks, wherein each pilot-symbol block comprises arespective plurality of pilot symbols and a respective guard interval;and a first set of one or more payload-symbol blocks, wherein eachpayload-symbol block comprises a respective plurality of constellationsymbols from the first sequence; and an optical modulator configured tomodulate an optical carrier to generate a modulated optical signal fortransmission of the data frame over an optical communication link,wherein: at least one payload-symbol block and another block areconcatenated without a guard interval between them; the other block iseither a pilot-symbol block or a payload-symbol block; the digitalprocessor is further configured to convert a second data stream into asecond sequence of constellation symbols, wherein the data frame furthercomprises: a second plurality of pilot-symbol blocks, wherein eachpilot-symbol block comprises a respective plurality of pilot symbols anda respective guard interval; and a second set of one or morepayload-symbol blocks, wherein each payload-symbol block of the secondset comprises a respective plurality of constellation symbols from thesecond sequence; the optical modulator is further configured to modulatea first polarization of the modulated optical signal with the symbols ofthe first plurality of pilot-symbol blocks and the first set of one ormore payload-symbol blocks; the optical modulator is further configuredto modulate a second polarization of the modulated optical signal withthe symbols of the second plurality of pilot-symbol blocks and thesecond set of one or more payload-symbol blocks; the first plurality ofpilot-symbol blocks comprises a first frame-synchronization block havinga first string of N pilot symbols concatenated with a second string of Npilot symbols, said second string being a copy of the first string,where N is a positive integer; the second plurality of pilot-symbolblocks comprises a second frame-synchronization block having a thirdstring of N pilot symbols concatenated with a fourth string of N pilotsymbols, said fourth string being a copy of the third string; and thethird string is different from the first string.
 19. An opticaltransmitter, comprising: a digital signal processor configured toconvert a first data stream into a first sequence of constellationsymbols and generates a data frame that comprises: a first plurality ofpilot-symbol blocks, wherein each pilot-symbol block comprises arespective plurality of pilot symbols and a respective guard interval;and a first set of one or more payload-symbol blocks, wherein eachpayload-symbol block comprises a respective plurality of constellationsymbols from the first sequence; and an optical modulator configured tomodulate an optical carrier to generate a modulated optical signal fortransmission of the data frame over an optical communication link,wherein: at least one payload-symbol block and another block areconcatenated without a guard interval between them; the other block iseither a pilot-symbol block or a payload-symbol block; the digitalprocessor is further configured to convert a second data stream into asecond sequence of constellation symbols, wherein the data frame furthercomprises: a second plurality of pilot-symbol blocks, wherein eachpilot-symbol block comprises a respective plurality of pilot symbols anda respective guard interval; and a second set of one or morepayload-symbol blocks, wherein each payload-symbol block of the secondset comprises a respective plurality of constellation symbols from thesecond sequence; the optical modulator is further configured to modulatea first polarization of the modulated optical signal with the symbols ofthe first plurality of pilot-symbol blocks and the first set of one ormore payload-symbol blocks; and the optical modulator is furtherconfigured to modulate a second polarization of the modulated opticalsignal with the symbols of the second plurality of pilot-symbol blocksand the second set of one or more payload-symbol blocks; the firstplurality of pilot-symbol blocks comprises: a first channel-estimationblock having a first polyphase sequence of symbols; and a secondchannel-estimation block having a second polyphase sequence of symbolsdifferent from the first polyphase sequence of symbols; and the secondplurality of pilot-symbol blocks comprises: a third channel-estimationblock having the second polyphase sequence; and a fourthchannel-estimation block having the first polyphase sequence, wherein:the first channel-estimation block is synchronous with the thirdchannel-estimation block; and the second channel-estimation block issynchronous with the fourth channel-estimation block.
 20. An opticaltransport system, comprising: an optical transmitter; and an opticalreceiver coupled to the optical transmitter via an optical communicationlink, wherein: the optical transmitter is configured to convert a firstdata stream into a first sequence of constellation symbols and generatesa data frame that comprises: a first plurality of pilot-symbol blocks,wherein each pilot-symbol block comprises a respective plurality ofpilot symbols and a respective guard interval; and a first set of one ormore payload-symbol blocks, wherein each payload-symbol block comprisesa respective plurality of constellation symbols from the first sequence;at least one payload-symbol block and another block are concatenatedwithout a guard interval between them, wherein the other block is eithera pilot-symbol block or a payload-symbol block; the optical transmitteris configured to modulate an optical carrier to generate a modulatedoptical signal having encoded thereon the data frame and to apply saidmodulated optical signal to the optical communication link; the opticalreceiver is configured to receive the modulated optical signal from theoptical communication link and to convert the received modulated opticalsignal into a first in-phase digital signal and a first quadrature-phasedigital signal that have a first set of signal samples corresponding tothe data frame; the optical receiver is configured to process the firstset of signal samples to recover the data encoded in the first sequenceof constellation symbols; the optical transmitter is further configuredto convert a second data stream into a second sequence of constellationsymbols, wherein the data frame further comprises: a second plurality ofpilot-symbol blocks, wherein each pilot-symbol block comprises arespective plurality of pilot symbols and a respective guard interval;and a second set of one or more payload-symbol blocks, wherein eachpayload-symbol block of the second set comprises a respective pluralityof constellation symbols from the second sequence; the opticaltransmitter is further configured to modulate a first polarization ofthe modulated optical signal with the symbols of the first plurality ofpilot-symbol blocks and the first set of one or more payload-symbolblocks; the optical transmitter is further configured to modulate asecond polarization of the modulated optical signal with the symbols ofthe second plurality of pilot-symbol blocks and the second set of one ormore payload-symbol blocks; the first plurality of pilot-symbol blockscomprises: a first channel-estimation block having a first polyphasesequence of symbols; and a second channel-estimation block having asecond polyphase sequence of symbols different from the first polyphasesequence of symbols; and the second plurality of pilot-symbol blockscomprises: a third channel-estimation block having the second polyphasesequence; and a fourth channel-estimation block having the firstpolyphase sequence, wherein: the first channel-estimation block issynchronous with the third channel-estimation block; and the secondchannel-estimation block is synchronous with the fourthchannel-estimation block.
 21. An optical communication method,comprising: converting a first data stream into a first sequence ofconstellation symbols; generating a data frame that comprises: a firstplurality of pilot-symbol blocks, wherein each pilot-symbol blockcomprises a respective plurality of pilot symbols and a respective guardinterval; and a first set of one or more payload-symbol blocks, whereineach payload-symbol block comprises a respective plurality ofconstellation symbols from the first sequence; and modulating an opticalcarrier to generate a modulated optical signal for transmission of thedata frame over an optical communication link, wherein: at least onepayload-symbol block and another block are concatenated without a guardinterval between them; the other block is either a pilot-symbol block ora payload-symbol block; and the first plurality of pilot-symbol blockscomprises a channel-estimation block having a first polyphase sequenceof symbols, which has been generated from a second polyphase sequence ofsymbols using one or more of the following: cyclically shifting thesecond polyphase sequence; phase shifting each symbol of the secondpolyphase sequence by a constant phase; taking an m-th power of eachsymbol of the second polyphase sequence, where m is an integer greaterthan one; and phase-conjugating the second polyphase sequence.
 22. Themethod of claim 21, wherein the second polyphase sequence has a constantamplitude and a phase pattern defined by phase sequence φ₁(n) or phasesequence φ₂(n), where φ₁(n)=−π(n−1)²/N; φ₂(n)=−πn(n−1)/N; n=1, 2, . . ., 2N; and N is a positive integer.
 23. An optical communication method,comprising: converting a first data stream into a first sequence ofconstellation symbols; generating a data frame that comprises: a firstplurality of pilot-symbol blocks, wherein each pilot-symbol blockcomprises a respective plurality of pilot symbols and a respective guardinterval; and a first set of one or more payload-symbol blocks, whereineach payload-symbol block comprises a respective plurality ofconstellation symbols from the first sequence; modulating an opticalcarrier to generate a modulated optical signal for transmission of thedata frame over an optical communication link, wherein: at least onepayload-symbol block and another block are concatenated without a guardinterval between them; and the other block is either a pilot-symbolblock or a payload-symbol block; and converting a second data streaminto a second sequence of constellation symbols, wherein the data framefurther comprises: a second plurality of pilot-symbol blocks, whereineach pilot-symbol block comprises a respective plurality of pilotsymbols and a respective guard interval; and a second set of one or morepayload-symbol blocks, wherein: each payload-symbol block of the secondset comprises a respective plurality of constellation symbols from thesecond sequence; a first polarization of the modulated optical ismodulated with the symbols of the first plurality of pilot-symbol blocksand the first set of one or more payload-symbol blocks; a secondpolarization of the modulated optical is modulated with the symbols ofthe second plurality of pilot-symbol blocks and the second set of one ormore payload-symbol blocks; the first plurality of pilot-symbol blockscomprises: a first channel-estimation block having a first polyphasesequence of symbols; and a second channel-estimation block having asecond polyphase sequence of symbols different from the first polyphasesequence of symbols; and the second plurality of pilot-symbol blockscomprises: a third channel-estimation block having the second polyphasesequence; and a fourth channel-estimation block having the firstpolyphase sequence, wherein:  the first channel-estimation block issynchronous with the third channel-estimation block; and  the secondchannel-estimation block is synchronous with the fourthchannel-estimation block.